1. Field of the Invention
The present invention relates to a printed circuit board (PCB), and more particularly to a PCB having vias for reducing reflections of input signals.
2. General Background
In modern communications, transmission speed of signals has become quicker and quicker. Signal integrity refers to fidelity of signal curves. However, qualities of signals are often reduced because of signal reflection that is caused by discontinuity of impedance in transmission lines of the PCB.
More signal transmission layers are needed because of the increasing density of signal lines. So it is inevitable that the signal transmissions between different layers of a PCB should be achieved through conductive vias. Design of the conductive vias is crucial in the design of multilayer PCBs. The vias can be divided into three types according to their usage and process of manufacture. They are blind vias, buried vias, and through vias. The blind vias are usually located on a surface of the top layer or bottom layer of the PCB to connect signals between surface layers and inner layers. The buried vias are located in the inner layers, and the through vias penetrate through all the layers of the PCB to complete electrical connections among all the layers. When a connection between the vias and transmission lines is necessary, discontinuity of impendence of transmission lines affects the integrity of the signals.
Referring to FIG. 1, a conventional via of a PCB comprises a drill hole 11, a pad 13, and an anti-pad 15. The drill hole 11 is a through hole. The pad 13 is located at opposite signal layers. So there are two same pads 13. The anti-pad 15 is located at non-connective layers. Referring to FIG. 2, a circuit parameter module of a via comprises two capacitances C0, C1 and an inductance L. The magnitude of the capacitance C0 or C1 is affected by sizes of the pad and the anti-pad. The approximate capacitance of each pad 13 is determined according to the following formula:
  C  =            1.41      ⁢              ɛ        r            ⁢              D        1            ⁢      T                      D        2            -              D        1            Wherein ∈r is the dielectric constant of the PCB, T is the thickness of the PCB layers to which the via is connected, D1 is the diameter of the pad, D2 is the diameter of the anti-pad. Referring to FIG. 1, in the prior art, D10=D12. Effects of an equivalent capacitance are greater than that of the inductance, therefore, the impedance of the whole via is counted as follows:
      C          via      ⁢                          ⁢      1        =                    C        10            +              C        12              =                  2        ×                  C          10                    =              2        ×                              1.41            ⁢                          ɛ              r                        ⁢                          D              10                        ⁢            T                                              D              20                        -                          D              10                                          
In the designing process, the size of the via is designed to be smaller to reduce the stray capacitance of the via.
Commonly in the design process, diameters of the pad and the anti-pad of the via are reduced simultaneously. Thereby, the reflection of the input signals is reduced because of a reduction in a capacitance of the corresponding pad. However, the diameter of the pad connected with the output signals is reduced as well, which results in a reduction in reflection of the output signals, therefore, the error of the output signals is larger which affects the signal integrity of the output signals. When the signals are transmitted, the capacitance of the pad connected with the input signal lines is large, so the impedance of the pad is small nearly to the point of being a short circuit, however, the signal lines have a certain impedance. Therefore, the impedance of the transmission lines are discontinuous, and large reflections are caused.
What is needed, therefore, is a PCB having vias designed for reducing the reflections of input signals.